Researchers at
UCSD and Clemson University have discovered that specially synthesized
carbon nanotube structures exhibit electronic properties that
are improved over conventional transistors used in computers.
In a paper published in the September issue of Nature Materials
and released online on August 14, UCSD engineering professors
Prabhakar Bandaru and Sungho Jin, graduate student Chiara Daraio,
and Clemson physicist Apparao M. Rao reported that Y-shaped
nanotubes behave as electronic switches similar to conventional
MOS (metal oxide semiconductor) transistors, the workhorses
of modern microprocessors, digital memory, and application-specific
integrated circuits.

“This is the
first time that a transistor-like structure has been fabricated
using a branched carbon nanotube,” said Bandaru. “This
discovery represents a new way of thinking about nano-electronic
devices, and I think people interested in creating functionality
at the nanoscale will be inspired to explore the ramifications
of these Y-junction elements in greater detail.”

The
stunning increase in the speed and power efficiency of electronics
over the past two decades was primarily due to the steady shrinkage
in size of conventional transistors. Chip makers have reduced
the minimum feature size of transistors to about 100 nanometers,
and that dimension is expected to shrink by the end of this
decade. However, industry experts predict that fundamental technological
and financial limits will prevent the makers of conventional
MOS transistors to reduce their size much further. However,
the Y-shaped nanotubes discussed in the Nature Materials
paper are only a few tens of nanometers thick and can be made
as thin as a few nanometers.

“The small size
and dramatic switching behavior of these nanotubes makes them
candidates for a new class of transistor,” said Bandaru.

The new transistors
were initially grown as straight nanotube elements. Titanium-modified
iron catalyst particles added to the synthesis mixture are then
attached to the straight nanotubes, nucleating additional growth,
which continued like branches growing from a tree trunk. Consequently,
the nascent nanotubes assumed a Y-shape with the catalyst particle
gradually becoming absorbed at the junction of the stem and
two branches.

When electrical contacts
are attached to the nanotube structures, electrons travel into
one arm of the Y, hop onto the catalyst particle, and then hop
to the other arm and flow outward. Experiments conducted in
Bandaru’s lab at UCSD’s Jacobs School of Engineering
showed that the movement of electrons through the Y-junction
can be finely controlled, or gated, by applying a voltage to
the stem. Bandaru hypothesized that positive charge applied
to the stem enhances the flow of electrons through the two arms,
producing a strong “on” signal. However, when the
polarity of the charge is reversed, the movement of electrons
through the arms essentially stops, creating an “off”
signal. Such binary logic is the basis of nearly all transistors.

“Among electrical
device engineers, this phenomenon is called gating,” said
Bandaru. He said the phenomenon effectively makes Y-shaped nanotubes
the smallest ready-made transistor yet, with rapid switching
speeds and possible three-way gating capability. In earlier
attempts to make carbon nanotube-based transistors, separate
gates were added rather than built in.

“We think this
discovery extends the paradigm of nanotechnology beyond just
making things small,” said Bandaru. “We can synthesize
functionality at the nanoscale, in this case to include the
three elements of a circuit – the gate, source, and drain
– and we don’t have to go to the trouble of making
them separately and assembling them.”

The researchers plan
to experiment with various other catalyst particles in order
to tailor the three-way gating properties of the Y-junctions.
“If we can easily fabricate, manipulate, and assemble
these nano-devices on a large scale they could become the basis
of a new kind of transistor and nanotechnology,” said
Bandaru.